1. Field of the Invention
This invention relates to a system for measuring the specific gravity of liquids, in particular, urine. This invention more particularly concerns an automated system for urinalysis, which includes a dispenser, a sample container in which the liquid is dispensed and flushed, and a fiber optic sensor system to record the refractive index of the liquid (a fiber optic refractometer).
2. Description of the Related Art
A. Specific Gravity
Specific gravity determinations serve a wide variety of purposes. This variable, obtained from various body fluids, particularly urine, is a part of almost all routine clinical diagnostic work-ups.
Specific gravity is a dimensionless term and relates, in the case of a solution, to the ratio of the weight (density) of a certain volume of the solution to that of an equal volume of a standard substance (e.g., water) at the same temperature. For solutions such as urine, specific gravity is related to the number, density, ionic charge and weight of the various species of dissolved solutes.
Specific gravity measurements are useful clinically because specific gravity alters as a function of abnormal states. For example, the specific gravity of urine varies as electrolyte disturbances occur. These disturbances accompany various diseases, for example, diabetes. Consequently, specific gravity values may be one indication of disease.
B. Clinical Analyses of Urine
Routine urinalysis, as practiced at the present time, involves three basic areas of investigation: a determination of the presence or absence of substances such as glucose, protein, occult blood, ketones, and so forth; a determination of specific gravity; and a microscopic examination of the urinary sediment. The first area of investigation usually involves the testing of the urine specimen with indicator papers or strips comprising reagent pads responsive to the urinary constituents to be determined. Indicator strips, usually in the form of single strips carrying multiple reagent pads responsive to the different urinary constituents to be determined, are dipped momentarily into the urine specimens, and the resulting color responses are compared to a color chart. Under present technology, separate analytical steps must be undertaken to determine urinary specific gravity and to microscopically examine the urinary sediment.
C. Specific Gravity of Urine
Urine is composed of various solutes in water (the solvent). Most of the solute in urine is non-liquid. The specific gravity of urine indicates the relative proportions of dissolved solid components to the total volume of the specimen tested, thus reflecting the relative degree of concentration or dilution of the specimen. Under appropriate and standardized conditions of fluid restriction or increased uptake, the specific gravity of a urine specimen measures the concentrating and diluting abilities of the kidney.
Because in urine, the solute consists of only dissolved solids, the refractive index (ratio of the phase velocity of light in a vacuum to that in a specified medium) of urine closely correlates with its specific gravity.
Normal human urinary specific gravity ranges from 1.003 to 1.035, but usually remains between 1.010 and 1.025. Specific gravities below 1.010 can be indicative of diabetes insipidus, a disease caused by the absence of, or impairment to, the normal functioning of the antidiuretic hormone. Low specific gravity may also occur in patients with glomerulonephritis, pyelonephritis, and various renal anomalies. Specific gravity is high in patients with diabetes mellitus, adrenal insufficiency, hepatic disease, and congestive cardiac failure. Therefore, urinary specific gravity determinations are useful in routine urinalysis as a screening procedure for detecting potentially abnormal clinical conditions. For veterinary applications, values of specific gravity extend up to about 1.08.
D. Methods of Measuring Urine Specific Gravity
Determination of urine specific gravity is of considerable clinical value in the understanding and clinical management of electrolyte disturbances. Therefore, complete urinalysis should, and usually does, include a specific gravity determination. This determination may be by direct or indirect methods.
Specific gravity may be measured directly or calculated from the measurement of a related property, e.g., osmolarity or ionic strength. Previous methods for determining specific gravity include use of hydrometers, urinometers, pycnometers, gravimeters, refractometers, and the like.
Three basic indirect measurements have been used to determine urine specific gravity:
a) the relative density measurable by a hydrometer which consists of a weighted float with a calibrated scale;
b) the refractive index, correlated in urine to the amount of dissolved solids, determinable by measuring the refractive index relative to water;
c) the osmolarity, wherein the dissolved solid content of urine is determined by measuring the temperature at which the sample freezes, based on the ability of solutes to lower the freezing point of water.
Other variations include a urinometer, a hydrometer adapted to measure the specific gravity of urine at room temperature. According to one method, a weighted float with a calibrated scale is immersed in at least 15 ml of urine. When the weight comes to rest, the meniscus of the urinometer is recorded. Adjustments must be made for room temperature, and for the presence of glucose or protein in the sample. The urinometer indicates the relative density of the sample which generally corresponds to the specific gravity of the sample.
The osmolarity/specific gravity of a sample can be determined by measuring the temperature at which the sample freezes. For example, a solution containing 1 osmol or 1000 mosm/kg water depresses the freezing point 1.86 degrees centigrade. Standard tables have been developed for comparing the measured freezing point to known osmolarities. From the determined osmolarity, the specific gravity can be estimated.
When only a small sample is available to test, the refractive index of the sample can be used to estimate the specific gravity. A refractometer is a laboratory instrument used to make this estimation. The refractometer measures the refractive index of the sample. The refractive index of the sample is related to the content of dissolved solids. Standard tables have been produced which correlate the refractive index of the sample with a known specific gravity.
There are also some instrumental applications which utilize the "falling drop" method in which the urine density is determined by the rate at which a drop of urine travels through an immiscible medium of known composition.
In the "falling drop" method, a drop of the sample is placed into an organic solvent, e.g., benzene, chloroform, and the speed at which the drop falls in the solvent is measured. Standard tables have been constructed which relate the speed of the sample drop through the solvent to the specific gravity of the sample. This method does not have a high degree of accuracy and is not suitable for large scale screening, or for very small amounts of sample, such as those routine in clinical practice.
Reagent strips made by Miles Labs (Ito, et al., 1983) correlate with refractometer readings on urine.
E. Problems in the Related Art
Most of the optical methods used to detect the refractive index of fluids, are based on exploiting the reflection and refraction phenomena which occur near the critical angle of light-liquid interface. They essentially consist of transmitting light through a transparent, light-conducting structure immersed in the fluid medium, so that light undergoes multiple internal reflections on the walls of the structure. The determination of the intensity of the light thus transmitted by multiple reflections, and the sudden variation of this intensity near the critical angle, thus permits the refractive index of the fluid to be determined. These instruments have many disadvantages. Many are fragile, yet bulky instruments, which require continuous cleaning, maintenance, and calibration to maintain their sensitivity and reliability. Some are hard to read because of meniscus must be viewed. In some urinometers, the sample adheres to the sides of the container holding the liquid sampler. Some, e.g., prisms for the transmission of light, are not accurate for turbid fluids.
Another problem for obtaining accurate readings is that the volume of urine obtained in the clinical sample may be inadequate for use of various measuring devices. Methods and devices are needed for analysis of small amounts of liquids, e.g., &lt;1 .mu.l, preferably about 250 .mu.l.
Most devices operate accurately only within a very narrow SG range and have problems for a wide range, for example, SG=1.00 to a least 1.09.
F. Patents on Devices to Measure Specific Gravity
Harmer has reported several variations on an optical fiber refractometer. Pointing out that "Optical fibres are particularly useful in that they may be used in inaccessible places . . . " p. 106 (Harmer, A. L., "Optical Fibre Refractometer Using Attenuation of Cladding Modes," Battelle Research Institute, Geneva Switzerland in the Proceedings of the First International Conference on Optical Fiber Sensors, London, Institute of Electrical Engineering (April, 1983)), the author uses as an application example, the "measurement of charge-state in lead acid batteries by monitoring refractive index changes." This reference points out the importance of the angles of light striking the probe-liquid interface to achieve optimum sensitivity. Harmer expresses a belief that the novelty of his refractometer is that the probe-liquid interface could be varied and controlled by introducing alternating bends in a multimode fibre.
Sensitivity increases as the refractive index of the fibre approaches that of the liquid but generally with a corresponding decrease in efficiency. In U.S. Pat. No. 4,187,025, a device is described for producing a light signal corresponding to the refractive index of a fluid medium. In this device, a curved section of the device is immersed in the fluid to be measured. The purpose of this sensor is to detect changes in the state of a fluid with improved sensitivity. It appears most applicable to continuous monitoring of large amounts of fluid rather than to single measures of small amounts of fluid, such as those routinely collected clinically. Multiple curvatures are used to increase sensitivity.
U.S. Pat. No. 4,015,462 relates to a system in which a carrier matrix is incorporated with osmotically fragile microcapsules, with a colormetric determination of osmolarity when the capsules are in contact with a solution. This is an example of an indirect measure of specific gravity.
Another test means, device and method for determining the ionic strength or specific gravity of an aqueous sample, makes use of a polymer salt and an indicator means capable of producing a detectable response to ion exchange between the polymer salt and the test sample (U.S. Pat. No. 4,473,650; see also U.S. Pat. No. 4,376,827, U.S. Pat. No. 4,532,216).
U.S. Pat. No. 4,433,913 describes a device for determining the index of refraction of a fluid, especially that in lead-acid storage batteries, employing a fiber optic sensor.
In an approach to measuring the refractive indices of both a sample medium and a reference medium to improve sensitivity, two sensors are combined in a single measuring portable probe which is connected by a flexible cable to a housing unit which accommodates the source of light, photodetectors, a test data processor, and an indicator. An advantage of this device is that concurrent readings of both a sample and a reference medium may be made under the same environmental conditions.
The invention disclosed in U.S. Pat. No. 4,427,293 comprises a double optical probe to determine the refractive index of liquids. The advantage of the double probe is to measure concurrently the liquid to be tested and a reference liquid, so as to avoid artifacts in the readings due to environmental fluctuations which might affect sequential readings. The reference must be read under the same conditions in order to obtain a correct estimate of the refractive index of the tested liquid.
U.S. Pat. No. 4,240,747 discloses fiber optic sensors having complex, alternating curvatures to measure the refractive index of a fluid medium. The device also has an improved filtering system to reduce the risk of contamination from suspension cultures. Improved sensitivity of measurements made by this device is attributed to various geometries of curves of sections of the sensors immersed in the fluid to be measured.
Most of the devices invented by Harmer (cited as patents above) disclose different shapes of the devices for light path production and detection systems. These shapes reported are presented as improvements over the basic, earlier versions of refractometers which comprise an optical detector, a light source, and a light-conductive structure connecting the detector with the source.
U.S. Pat. No. 4,076,052 describes a method, composition and device for determining and physical readings that are functions of specific gravity.
U.S. Pat. No. 4,318,709 is directed to a means and a method for determining the ionic strength or specific gravity of a fluid sample.
U.S. Patent No. 4,376,827 describes the use of pH indicators to make a determination which is proportional to the ionic strength of a solution. The ionic strength is used to approximate specific gravity.
U.S. Pat. No. 4,318,709 uses test strips (?) for determining ionic strength or specific gravity of an aqueous test sample.
U.S. Pat. No. 4,639,594 presents a fiber optic probe for in situ sensing of liquid level, concentration and/or phase change which attempts to solve the problem of temperature standardization of the sample and the reference liquid, by pulling the reference liquid inside the probe.
U.S. Pat. No. 4,564,292 presents a refractometer representing an improvement over the prior art wherein the previously separated sample medium sensor and the reference medium sensor are merged into a single measuring probe.
G. Status of the Art
Because the determination of whether to treat a patient based on a urinalysis is often done on an outpatient basis, and because only small amounts of fluid may be present, quick and accurate results for small specimens are needed to institute treatment while the patient is present.
As reviewed in the previous sections, the specific gravity of a liquid is a measure of the density of the liquid with respect to water. Further, the refractive index of a liquid (an optical property) is also dependent on the density of the liquid. Therefore, a measure of the refractive index has been used to infer the density (i.e., specific gravity) of a liquid. A fiber optic sensing concept has been suggested for measuring specific gravity; however, complete systems have neither been suggested nor successfully developed for small volumes of liquid, in particular, as obtained for multiple tests in a clinical setting.